1. Technical Field of the Invention
This invention most generally relates to methods and apparatus for drying coffee beans and other green crops or coarse granular bulk materials composed of beans, seeds, pods, or grains of relatively large and uniform size, to reduce the moisture content. Most particularly, it relates to convective airflow drying systems, especially solar powered system, and removable, internally ventilated containers for handling the bulk materials being dried.
2. Background
The art of harvesting and processing coffee beans from tree-borne cherries to the green coffee bean of commerce consists of two principle methods, the "dry" method and the "wet" method. Either method must result in moisture content equivalent to one third or more of the bean's weight being removed, to produce a commercial product.
The dry method is the more ancient and rudimentary. The cherries are hand-picked all in one picking, washed, and sun-dried on drying ground or concrete slabs in thin layers, usually for a period of two to three weeks. The beans ferment during the process, and are turned several times a day to promote even drying. They are covered at night to protect them from moisture.
In the wet method, only the ripe cherries are picked in any one picking of a tree. It may take three to five sequential pickings in a season over the time it takes between the earliest and the latest cherries to ripen. After the cherries are washed, the outside fruit pulp is removed by machines and the berries are then placed into large concrete tanks to ferment for twelve to twenty-four hours, then poured into concrete sluiceways or washing machines to be thoroughly washed in constantly running water. Then they are dried in much the same way as in the dry method, except that the drying time is shorter. These beans are then processed through hulling machines to remove the remaining layers of skin.
Problems with either method of this art include the inefficient, labor-intensive and lengthy sun-drying time of beans on concrete slabs or the use of other manual, passive solar methods and devices. There are many patents that describe related technologies and devices. Various fuel-fired dryer systems have been used to try to accelerate the drying time and prevent the mold problems. These alternatives add expense and complexity to an otherwise simple process. Failing to safeguard the beans from excess moisture, in particular the formation of mold during the drying process is crucial as the value of the crop drops dramatically if mold occurs. Overdrying can also occur using accelerated methods; this also affects the quality and value of the crop.
The requirements for loading and unloading the crop into and out of small, stand alone, solar dryer systems is burdensome. The passive solar drying time is always too long; leaving the crop exposed to potential downturns in weather conditions.